FIG. 6 is a schematic block diagram showing a prior art voltage/current characteristic measuring unit (PMU) employed in conventional semiconductor testing apparatus (for instance, a semiconductor characteristic measuring apparatus such as an HP4145 manufactured by the Hewlett Packard Company). The unit is capable of performing both voltage setting/current measuring operations and current setting/voltage measuring operations, the architecture of which is widely used in IC tester and DC characteristic evaluation apparatus marketed by the Applicant's Assignee.
In FIG. 6, an error amplifier 111 is connected via an integrator 112 and a buffer 113 to one terminal "a" of a current measuring resistor 120. Signal generating source 110 comprises error amplifier 111, integrator 112, and buffer 113.
The other terminal "b" of current measuring resistor 120 is connected to a predetermined terminal of a DUT. Both ends of resistor 120 are connected via buffers 131a and 131b, respectively, to input terminals of a differential amplifier 132. An output terminal of differential amplifier 132 and an output terminal of buffer 131b are connected to error amplifier 111.
It should be noted that the buffers 131a, 131b and differential amplifier 132 constitute a current measuring circuit, whereas the buffer 131b constitutes a voltage measuring circuit. When voltage setting/current measuring operations are performed, a setting voltage (V.sub.FIN) is applied as an analog voltage to error amplifier 111 from a measuring signal process circuit (not shown) via a digital to analog converter (DAC) (not shown). Error amplifier 111 receives a voltage V.sub.MOUT appearing at terminal "b" of current measuring resistor 120, compares voltage V.sub.FIN with voltage V.sub.MOUT appearing at the terminal "b", and outputs an error signal to the integrator 112 in order that V.sub.MOUT becomes equal to V.sub.FIN.
A current (namely, a current supplied to a DUT) flowing through the current measuring resistor 120 may be detected by measuring a voltage across ends "a" and "b" of resistor 120. The voltage across ends "a" and "b" is provided as an output voltage from differential amplifier 132 and is then supplied via an ADC (not shown) to the above-described measurement signal processing circuit.
When current setting/voltage measuring operations are executed, a setting current signal (I.sub.FIN) is supplied from the above-mentioned measurement signal processing circuit via a DAC to error amplifier 111. Error amplifier 111 is fed the voltages appearing across both ends of resistor 120, and outputs an error signal to integrator 112 in order that a current (namely, the current supplied to a DUT) flowing through resistor 120 becomes equal to the setting current. A voltage applied to the DUT is detected by measuring a voltage at terminal "b" of resistor 120. This voltage is furnished via buffer 131b and an ADC to a measurement signal processing circuit (not shown).
However, since stabilities of the voltage and current feedback loops are established with respect to predictable load conditions in the above-described circuit, the gain/frequency characteristic of the overall feedback loop is suppressed to a low value. As a consequence, a lengthy time period is required until the output signal V.sub.OUT converges, and therefore it is difficult to accomplish a high-speed measurement.
Even if a load causing no stability problem is employed, because the feedback loop is activated based on worst case conditions, measurement speed is limited.
In other words, when the gain/frequency characteristic of the overall loop is extended to a high frequency range to achieve a high-speed measurement, overshoots occur when voltage settings are changed. A ringing phenomenon thus occurs, depending upon load conditions. In a worst case, other problems such as oscillations may occur. Also, since capacitors must be connected to various circuit portions to maintain stability of the feedback loop, errors caused by dielectric absorption and capacitive leakage currents occur.
The present invention solves the above-described problems, and therefore has as an object to provide a voltage/current measuring unit and a voltage/current measuring method, which can measure various electric characteristics (such as a voltage/current characteristic of DUT) at high-speed and under stable conditions.